Furnaces – Process – Treating fuel constituent or combustion product
Reexamination Certificate
2001-09-28
2004-04-20
Langel, Wayne A. (Department: 1754)
Furnaces
Process
Treating fuel constituent or combustion product
C423S235000, C423S239100, C423S243010, C423S243060, C423S243080, C423S244010, C423S244070, C423S244080
Reexamination Certificate
active
06722295
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention discloses a method for the simultaneous reduction of the concentrations of sulfur dioxide, SO
2
, and nitrogen oxides, NO
x
, in the products of combustion of a fossil fuel. The method consists of injecting into the combustion gas stream, liquid droplets containing lime or very fine limestone particles dispersed in water and urea or ammonia dissolved in the same water. Said particle dispersion and solution are produced in a supply vessel by continuous mixing of the lime or very fine limestone solids and the urea solids or ammonia liquid in concentrations of up to 30% by weight for the lime or very fine limestone and up to 10% by weight of the urea or ammonia in the water. Under certain conditions, such as preparing the mixtures in large vessel in which the mixture will remain for extended periods of time, it may be desirable to augment the continuous mixing by the addition of a surfactant and/or stabilizer in order to maintain a uniform dispersion of the lime or very fine limestone particles. Injection of this mixture takes place in a furnace in a temperature range from about 1700° F. to 2200° F., where both the calcination of lime or very fine limestone and the subsequent reaction of calcium oxide with SO
2
and the reaction of urea or ammonia molecules with NO
x
, are effective. Specifically, the method disclosed consists of preferably using air atomized water droplet injectors that are designed to disperse said droplets exclusively in the optimum gas temperature zone at which vaporization of the droplets disperses the lime or very fine limestone particles and the urea or ammonia gas molecules throughout said gas zone where the SO2 and NO
x
reduction reactions are effective.
Coal is the primary fuel for utility boilers, and to efficiently burn it requires combustion at 3000° F. or higher. Very extensive deposits of high sulfur coals that contain fuel bound nitrogen are available in the Eastern half of the United States, and the use of this coal, especially in the mid-Western States is a major source of SO
2
and NO
x
pollution in the Eastern United States.
The combustion of fossil fuels leads to the formation of NO
x
and SO
2
, pollutants that lead to smog and acid rain over wide areas far removed from the combustion source, and it is especially a problem in urban environments. There are two sources of NO
x
, one is primarily formed during the combustion of solid fossil fuels, namely coal. The fuel bound nitrogen whose concentration is generally in the range of 1%, by weight in the coal is the primary source of NO
x
in coal combustion. Additionally, combustion with oxygen in excess of the amount required for stoichiometric combustion, which is required for all fossil fuels to minimize other pollutants, such as carbon monoxide, results in the formation of thermal NO
x
. The thermal NO
x
concentration rises substantially at temperatures above about 3000° F.
Several technologies are used to control the emissions of NO
x
from fossil, and especially from coal, fired boilers. Among these control technologies are: staged combustion in which initial fuel rich-combustion near the fuel injection zone is followed by excess air combustion in the furnace region of the boiler. There are a number of different staged combustion processes and system designs, depending on the boiler design. Another NO
x
control process is selective catalytic reduction, SCR, in which the relatively cold combustion gas effluent from a boiler of several 100° F., is passed over a catalyst coated bed in the presence of ammonia. Another process, generally called selective non-catalytic reduction, SNCR, involves the injection of various chemical compounds, primarily urea or ammonia, with or without various chemical additives, into the combustion gases in the boiler furnace at temperatures at which the NO
x
to N
2
reaction is favored. The method of the present invention falls within the field of SNCR processes. While all these NO
x
control processes reduce NO
x
emissions to varying degrees, they all have certain technical and economic disadvantages. For example, staged combustion results in unburned carbon in the fly ash, which represents an energy loss and may make the fly ash unsuitable for recycling. Also in a certain staged combustion design, called low NO
x
burners, chemical compounds can form that corrode boiler metal tubes. SCR requires costly catalyst structures, and regular catalyst replacement. The present invention utilizes a SNCR method. It incorporates key elements of Zauderer's prior invention, (U.S. Pat. No. 6,048,510, herein incorporated by reference in its entirety) in that it eliminates some of the technical disadvantages in the prior invention by assuring a simpler and more uniform method of introducing the urea into the hot combustion gases and it shows that urea is preferred to ammonia for this process. These improvements are the result of practicing the art disclosed in the prior invention that came to light during subsequent testing, and that are disclosed in a subsequent invention by Zauderer on NO
x
control (U.S. Provisional Application No. 60/185,753, herein incorporated by reference in its entirety). Among the latter improvements are means to eliminate the overheating of the droplet injectors that inserted in the nominal 2000° F. combustion gas being treated for NO
x
reduction.
The combustion of these fossil fuels also leads to the formation of SO
2
, and both pollutants lead to smog and acid rain over wide areas far removed from the combustion source, and it is especially a problem in urban environments. Sulfur is widely present in coal at concentrations ranging from less that 1% to well above 4%, in some oils, and in some natural gases and oils. It reacts with oxygen during the combustion process to form SO
2
.
The SO
2
molecules that are formed during the combustion of a sulfur containing fossil fuel will react with calcium oxide particles dispersed in the combustion gas to form calcium sulfate, CaSO
4
. The sulfur gas capture reaction is preceded by calcinations of the lime, Ca(OH)
2
, or very fine limestone, CaCO
3
, in the hot combustion gases to form a very porous, reactive calcium oxide, CaO, particle. Calcination is essentially complete at temperatures of about 1800° F. It is followed by reaction of the CaO particle with the SO
2
gas molecules. Depending on the particle size and its residence time at temperatures considerably higher than 2000° F., the CaO particle overheats and begins to fuse. This fusing effect closes its porous structure and sharply reduces the effectiveness of the SO
2
capture reaction. Furthermore, at temperatures substantially higher than 2000° F., the CaSO
4
reaction reverses and the sulfur is re-evolved from the particle as a gas. It is, therefore, essential to implement the calcination and sulfur capture reactions at the appropriate temperature. The droplet method disclosed in this invention for introducing SO
2
capture reactants results in a most efficient and low cost method of implementing these process steps.
By coincidence the reaction of urea or ammonia vapor molecules with the NO
x
that converts the latter to nitrogen, N
2
, occurs under equilibrium conditions that overlap the temperature range of 1700° F. to 2200° F. at which the reaction of calcined lime or very fine limestone with SO
2
molecules is effective. Consequently, both processes can be implemented in the same apparatus. More importantly, the droplet method disclosed in this invention for introducing both NOx and SO2 capture reactants results in a most efficient and low cost method of implementing these processes.
While ammonia is somewhat more effective in reducing NO
x
, and less costly than urea, ammonia's toxicity and handling problems, as well as its high vapor pressure which can result in vaporization of the ammonia in the aqueous feed pipe leading to the injector and resulting in an unsteady, fluctuating flow, makes urea the preferred material for the present invention
There are a number of processes f
Langel Wayne A.
McGuireWoods LLP
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